This invention relates to piezoelectric composite materials having improved sensitivity for use as transducers and a method of making.
Piezoelectric materials develop a voltage when a pressure or stress is applied to them. The materials are used to fabricate pressure transducers for applications such as microphones and keyboards. Certain types of piezoelectric materials have been considered for use in underwater microphones, hydrophones, because of their substantially hydrostatic response.
Present hydrophones use pressure compensated ceramic lead zirconium titanate (PZT) cylinders. While pressure compensated PZT hydrophones offer substantially pressure independent response, such hydrophones are brittle and costly to fabricate for passive sonar applications. Uncompensated PZT discs have too low a voltage output in response to hydrostatic pressure to be useful.
PZT composite materials, i.e. PZT crystals dispersed in a polymer matrix, are flexible but have only moderately high voltage output in response to hydrostatic pressure. Polyvinylidene difluoride (PVF.sub.2) is a potentially useful hydrophone sensor material. However PVF.sub.2 is costly to manufacture and limited in thicknesses available. This is a severe limitation for hydrostatic sensor materials since their voltage output in response to sound increases linearly with thickness.
U.S. Pat. No. 2,420,864 to Chilowsky describes a class of piezoelectric plastic materials. In this patent, particles of piezoelectric materials such as tourmaline and tartaric acid are suspended in a polymer matrix. The particles of these piezoelectric materials obtain an electric dipole when a hydrostatic pressure is applied. The orientation of the particles in the polymer matrix is accomplished by applying an electric field simultaneously with pressure, producing a poled piezoelectric composite material. However, composites made according to prior art methods exhibiting substantial hydrostatic response, do not have sufficient stability over time.
U.S. Pat. 4,407,054 to Zipfel describes a method of making flexible piezoelectric composites exhibiting a stable hydrostatic response at room temperature over time. Stable response is achieved by choosing a polymer having a shear modulus of less than 20 psi. While Zipfel's method improves the stability of the piezoelectric composite, the sensitivity of the composite is not comparable to that of the pressure compensated PZT sensors. For piezoelectric composites, sensitivity is proportional to the hydrostatic voltage constant. Present piezoelectric composites achieve a hydrostatic voltage constant which is substantially less than that of the single crystal alone.
Therefore, it is an object of the present invention to provide a piezoelectric composite material having improved hydrostatic voltage constant.
It is another object of the present invention to provide a piezoelectric composite material that is stable over time.
It is also an object of the present invention to provide a piezoelectric composite having an improved hydrostatic sensitivity.
It is yet another object of the present invention to provide a method of fabricating a piezoelectric composite material having improved hydrostatic voltage constant and sensitivity.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.